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1. Field of the Invention
This invention relates to the field of digital moving image editing and more particularly the invention relates to a technique for automating the creation of complex mattes to generate rotoscopes.
2. Description of the Related Art
Rotoscoping is a technique for generating special effects shots in movies and other moving image media. One use of rotoscoping is to take footage shot in one context, say of a runner in a stadium, and place it in a different context, such as the banks of the Potomac River. Rotoscoping was used to create complex space and combat scenes in Star Wars. Traditionally rotoscoping required that an animator or other artist hand draw mattes that indicated the areas of interest in each frame of a moving image clip. The mattes would then be used to composite that clip with a second clip. The more complex the object being rotoscoped, the more work that was involved for the animator. Further, one hand drawn matte is necessary for every frame of a sequence of moving images. With 30 frames per second in a movie, the task of hand drawing mattes quickly becomes time consuming and expensive.
Various methods have been used to make rotoscoping less time consuming and less expensive. Some of these methods have employed computer systems. Typically, such systems allow a single matte to be defined on a frame-by-frame basis. More sophisticated systems allow simple interpolation of that matte between frames and reduce the need for the animator to reposition the matte at each frame. These techniques are fairly brittle and typically cannot handle complex scenes without significant human intervention. For example, simple interpolation techniques used on some computer systems cannot capture rotation and scaling effects such as the motion of a sword knocked out of a combatant""s hand spinning into the background, thus the animator would need to relocate the matte on each frame. Further, the end product of these systems, as well as the traditional manual techniques, can be aesthetically unpleasant. That is to say the matting effect maybe visible in the final product, especially in areas that have fine details.
The inability of earlier computer systems to easily create multiple mattes with different properties limits the usefulness of the computer system in creating a rotoscope for a scene. For example, if the runner being matted passes behind a tree, a separate matte must be created for the tree. Previously, you could not easily describe multiple mattes in the computer system so that one matte would be considered in front of another matte. Further, the computer systems could not easily handle the disappearance of a matted object if it went out of the frame.
Another limitation of early computer systems is their inability to simulate motion blur. The objects being rotoscoped are typically moving and as a result a strobe like effect may be perceived. To prevent this effect, the composite clip can have a motion blur applied to it. The faint image of the matted object can reduce the strobe effect and reinforce the appearance of motion.
Prior techniques required tedious manual construction of a matte for each frame. Prior automated techniques only allowed the definition of a single matte and did not handle motion blur, feathering, rotation, and scaling of the matte. Accordingly, there is a need for an improved method for automated region based moving image editing.
A digital moving image editing system that permits the generation of complex mattes for rotoscoping is described.
One embodiment uses a computer system to define areas with moving image information of interest in a sequence of digital moving image frames. The sequence of digital moving image frames has at least three frames, a first frame, a second frame and a third frame, in time sequence. A set of points identifies the areas with moving image information of interest on the first frame. In the third frame, the areas with moving image information of interest are identified with the set of points. Each point in the set of points has an explicitly defined location in the first frame and an explicitly defined location in the third frame. The location of each point in the first frame corresponds to the same areas with moving image information of interest as each point""s location in the second and third frames. For example if one of the points is located at the tip of a runner""s left toe in the first frame, then that point will be located at the tip of the runner""s left toe in both the second and third frames. The computer system automatically computes the location of the areas of interest in the second frame.
In some embodiments, signals are received specifying velocity information for each point of the set of points. The velocity information permits the determination of the areas of interest in the frames in between the first and third frames to be computed with greater precision.
In some embodiments, the edges of the area are blurred either inward or outward to soften the edges of the area.
In some embodiments, multiple sets of points are used to define multiple areas of interest.
In some embodiments, a motion blur effect can be added. This effect improves the visual appearance of the final moving image images.